m 
MANSFIELD: POST-PLEISTOCENE DRAINAGE. 71 
545) speak of the outer gravel deposits as possibly of shore formation. 
The former (p. 298) thinks it “almost necessary to assume the occur- 
rence of an extensive lake surrounding the Hills during the Quater- 
nary period, when boulders resulting from erosion were transported 
by the agency of floating ice to the places where they now are found.” 
Even granting the assumption that the boulders were transported by 
floating ice, it does not seem to the writer necessary to hypothecate a 
lake to float the ice blocks, since in the spring season of high water 
large streams would be able to transport ice cakes of considerable size 
great distances. So far as his observations have gone all the facts of 
distribution seem well explained by the hypothesis of stream washing. 
No wave-cut benches or cliffs were observed by the writer and none are 
definitely described by either Jenney or Darton. The deposits seem 
to have extended a considerable distance eastward and also south- 
ward and westward; for Darton finds them capping portions of the 
Big Bad Lands and also lying on slopes adjoining Cheyenne River 
and Beaver Creek valleys (a, p. 547). He refers to them, however, 
always as gravels and does not suggest any gradation, such as might 
be expected in lacustrine deposits of great extent, into finer and more 
uniform deposits (Davis b, p. 352). Moreover he does not delimit 
the lake in any way, except by the general reference to the Pleistocene 
shore-line already quoted. 
Question of Uplijt. According to Darton (a, p. 558) the Black Hills 
dome was uplifted and truncated in early Tertiary time so that the 
larger topographic features of the region were developed before the 
deposition of the White River beds. A good idea of the degree of 
truncation may be obtained by consulting his sections (a, p. 550; see 
also c, the Newcastle folio). He speaks of widespread planation of 
the Tertiary deposits during the early Pleistocene period, of the revival 
of old valleys and the rearrangement of drainage on the east side of 
the hills, due to an increased tilting to the northeast during the late 
Tertiary uplift. In evidence of this last he cites the offsetting of Pre- 
Oligocene valleys northward through canyons of Post-Oligocene age, 
while saddles mark the previous courses of the valleys. He further 
states that some of the offsetting in the present drainage has been 
largely increased by early Pleistocene erosion and recent stream rob- 
bing and that there appears to have been further uplift in late Pleisto- 
cene time, “for the present valleys below the level of the earlier 
Pleistocene high level deposits seem to be cut deeper than would 
result simply from the natural progression of a lower plane of base- 
levelling up the Missouri and Cheyenne Rivers.” 
